W3Schools Tryit Editor

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<style>
* {
  box-sizing: border-box;
}

.responsive-form {
  max-width: 600px;
  width: 100%;
  border-radius: 4px;
  border: 2px solid #04AA6D;;
  background-color:  #f2f2f2;
  padding: 20px;
}

form {
  display: flex;
  flex-wrap: wrap;
  justify-content: space-between;
}

label {
  min-width: 48%;
  margin-bottom: 10px;
}

input[type=text], select {
  min-width: 45%;
  margin-bottom: 10px;
  border: 1px solid #ccc;
  border-radius: 4px;
  padding: 10px;
}

textarea {
  min-width: 45%;
  height: 150px;
  margin-bottom: 10px;
  border: 1px solid #ccc;
  border-radius: 4px;
  padding: 10px;
}

input[type=submit] {
  width: 100%;
  background-color: #04AA6D;
  color: white;
  padding: 14px;
  margin: 8px 0;
  border: none;
  border-radius: 4px;
  cursor: pointer;
}

@media screen and (max-width: 600px) {
  label,
  input[type=text],
  input[type=submit],
  select, textarea {
    flex: 100%;
    min-width: 100%;
  }
}
</style>
</head>

<body>
<h2>Responsive Form</h2>
<p>Resize the screen to see the form layout change!</p>

<form action="/action_page.php">
   <label for="fname">First Name:</label>
   <input type="text" id="fname" name="fname" placeholder="Your first name">

<label for="country">Country:</label>
   <select id="country" name="country">
     <option value="australia">Australia</option>
     <option value="canada">Canada</option>
     <option value="usa">USA</option>
    </select>
    
    <label for="message">Message:</label>
    <textarea id="message" name="message" placeholder="Write something.."></textarea>

</div>

</body>
</html>

​x
 
<!DOCTYPE html><html><head><meta name="viewport" content="width=device-width, initial-scale=1.0"><style>* {  box-sizing: border-box;}​.responsive-form {  max-width: 600px;  width: 100%;  border-radius: 4px;  border: 2px solid #04AA6D;;  background-color:  #f2f2f2;  padding: 20px;}​form {  display: flex;  flex-wrap: wrap;  justify-content: space-between;}​label {  min-width: 48%;  margin-bottom: 10px;}​input[type=text], select {  min-width: 45%;  margin-bottom: 10px;  border: 1px solid #ccc;  border-radius: 4px;  padding: 10px;}​textarea {  min-width: 45%;  height: 150px;  margin-bottom: 10px;  border: 1px solid #ccc;  border-radius: 4px;  padding: 10px;}